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Re: RAID card recommendation

From: "Gurgel, Flavio" <flavio(at)4linux(dot)com(dot)br>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: RAID card recommendation
Date: 2009-11-24 17:59:27
Message-ID: 7990482.115011259085567295.JavaMail.root@mail.4linux.com.br (view raw or flat)
Thread:
Lists: pgsql-performance
----- "Richard Neill" <rn214(at)cam(dot)ac(dot)uk> escreveu:

> Matthew Wakeling wrote:
> > 
> > We're about to purchase a new server to store some of our old
> databases, 
> > and I was wondering if someone could advise me on a RAID card. We
> want 
> > to make a 6-drive SATA RAID array out of 2TB drives, and it will be
> RAID 
> > 5 or 6 because there will be zero write traffic. The priority is 
> > stuffing as much storage into a small 2U rack as possible, with 
> > performance less important. We will be running Debian Linux.
> > 
> > People have mentioned Areca as making good RAID controllers. We're 
> > looking at the "Areca ARC-1220 PCI-Express x8 SATA II" as a
> possibility. 
> > Does anyone have an opinion on whether it is a turkey or a star?
> > 
> > Another possibility is a 3-ware card of some description.
> > 
> 
> Do you actually need a RAID card at all? It's just another point of 
> failure: the Linux software raid (mdadm) is pretty good.
> 
> Also, be very wary of RAID5 for an array that size. It is highly 
> probable that, if one disk has failed, then during the recovery
> process, 
> you may lose a second disk. The unrecoverable error rate on standard 
> disks is about 1 in 10^14 bits; your disk array is 10^11 bits in
> size...
> 
> We got bitten by this....
> 
> Richard

Linux kernel software RAID is fully supported in Debian Lenny, is quite cheap to implement and powerful.
I would avoid SATA disks but it's just me. SAS controllers and disks are expensive but worth every penny spent on them.

Prefer RAID 1+0 over RAID 5 not only because of the risk of failure of a second disk, but I have 3 cases of performance issues caused by RAID 5.
It's said that performance is not the problem but think twice because a good application tends to scale fast to several users.
Of course, keep a good continuous backup strategy of your databases and don't trust just the mirroring of disks in a RAID fashion.

Flavio Henrique A. Gurgel
Consultor -- 4Linux
tel. 55-11-2125.4765
fax. 55-11-2125.4777
www.4linux.com.br


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